Poly(3-hydroxyalkanoates) are naturally occurring thermoplastic polymers currently isolated from bacteria and other microorganisms. Unlike many thermoplastic polymers, poly(3-hydroxyalkanoates) and in particular poly(3-hydroxybutyrate) have been shown to be biodegradable and environmentally non-persistent. Poly(3-hydroxyalkanoates) have the additional feature of being both thermally and hydrolyticly unstable and thus can degrade without microbial intervention. However, the high cost of processing and isolating poly(3-hydroxyalkanoates) derived from natural sources has inhibited the wide spread use of these biodegradable polymers in commercial applications. A low cost synthetic method for the preparation of poly(3-hydroxyalkanoates) is, therefore, highly desirable.
Previous attempts at the production of poly(3-hydroxyalkanoates) by the polymerization of .beta.-substituted-.beta.-propiolactones generally fall into two categories, acid catalyzed reactions generally characterized by the use of Lewis acid catalysts containing metals such as aluminum or zinc, or anionic ring opening polymerizations. Examples of acid catalyzed polymerization of .beta.-substituted-.beta.-propiolactones can be found in the work of Tani et al. (Macromolecules, 10, 275 (1977) and Lenz et al. (Macromolecules 21, 2657, (1988).
The Lewis acids most frequently used in the acid catalyzed polymerizations have included triethyaluminium/water complexes (often referred to as aluminoxanes) and diethyl zinc/water systems as well as transition metal alkoxides such as aluminum triisopropoxide.
Examples of anionic ring opening polymerization of .beta.-substituted-.beta.-propiolactones can be found in the work of Tani et al. (Macromolecules, 10, 275 (1977), Kricheldorf et al. (J.Macromol. Sci.-Chem, A26, 951 (1989), and the work of Jedlinski et al. Macromolecules, 24,349, 1991). These references generally disclose that anionic ring opening polymerization occurs via nucleophilic attack at the .beta.-carbon of the .beta.-substituted-.beta.-propiolactones. However, this type of polymerization is slow and produces low molecular weight polymers. Tani et al. also disclose that the anionic ring opening polymerization of .beta.-alkyl or .beta.-haloalkyl-.beta.-propiolactones using conventional anionic catalysts either results in no ring opening or the termination of propagation at a very early stage.
Kricheldorf et al. disclose that, in solution or in bulk, non-ionic bases or ionic bases either result in no reaction or cause significant chain termination (often indicated by crotonate end group formation) during the ring opening polymerization of .beta.-butyrolactone. Kricheldorf et al. isolated only low molecular weight polymer using reaction temperatures of 50.degree. C. and reaction times of 48 hours.
Jedlinski et al. using potassium naphthalide in terahydrofuran solution in presence of 18-crown-6 or cryptand [2, 2, 2] produced low molecular weight poly(3-hydroxybutyrate) from .beta.-butyrolactone at room temperature with reaction times ranging from 96 to 200 hours. Using reaction times of 80 to 100 hours, Jedlinski et al. also demonstrated the preparation of low molecular weight poly(3-hydroxybutyrate) from .beta.-butyrolactone using potassium acetate/18-crown-6 or potassium crotonate/18-crown-6 as a polymerization initiator.
In light of the above it would be very desirable to be able to produce poly(3-hydroxyalkanoates) from .beta.-substituted-.beta.-propiolactones of higher molecular weights at increased reaction rates and higher yields.